Your phone or satnav receiver
routinely picks up signals from navigation satellites in order to tell
you precisely where you are. But have you ever thought what happens to
those satnav signals afterwards? A foresighted ESA inventor had the
idea of using them as a tool for observing the Earth. 1)

There are currently more than 120
satellite navigation satellites in orbit, making up multiple
constellations including Europe’s system, sending down a
continuous rain of satnav signals for the benefit of users worldwide.
Just like visible light, these microwave signals go on to reflect off
Earth’s land and sea surfaces.

The
traditional attitude to these reflected signals is to see them as
something of a nuisance – known in the trade as
‘multipath’, they can confuse satnav receivers and reduce
their overall accuracy.

But back in 1993 – at the same
time as the US GPS satnav system reached its full constellation of 24
satellites – a young ESA microwave engineer, named Manuel
Martin-Neira, came up with the idea of treating these satnav
reflections as a scientific resource instead. 2)3)4)

“My head of division asked me
to come up with a budget-friendly way of increasing the overall
sampling rate to build up a fuller picture of ‘mesoscale’
phenomena, and that led me to start looking into making use of
additional signals of opportunity, chiefly satnav signals.

“The initial reaction was
mixed, because the forecast accuracy was not as precise as the ERS-1
altimeter of ESA could deliver – but on the plus side there would
be a lot of these signals to make use of, and the performance has
improved a lot since those early days.”

The basic idea of what Manuel christened PARIS (Passive Reflectometry and Interferometry System)
comes down to a two-sided antenna. As the topmost side picks up a
satnav signal from the satellites in orbit, the other side picks up the
version of the signal bounced back from Earth.

By comparing this initial, overhead
signal with its reflected equivalent using a process called
interferometry – measuring tiny differences in signal phases
– the extra travel time of this reflected beam can be determined,
down to an accuracy of less than five centimeters, determining sea
height and sea ice thickness.

Additional ‘amplitude
waveform’ processing can deliver further data on wind and wave
measurements over the ocean, and soil moisture and biomass over land.

‘Satellite reflectometry’ has since grown into a thriving field. This summer Manuel attended the latest international workshop on the method he first devised 26 years ago.

Figure 3: The Passive
Reflectometry and Interferometry System (PARIS) concept involves a
dedicated constellation of satellites picking up reflected satnav
signals from GPS, and other navigation satellite constellations to
gather data on Earth's sea and land surfaces. Operating on the same
basis and in a similar way as current radar altimeters and
scatterometers, these returning signals can be used to build up global
maps of sea-surface height and wind and wave measurement over the
ocean, determine ice extent and thickness of the icecaps and indicate
soil moisture and biomass across land (image credit: ADS-SAU)

Figure 4: Workshop participants.
The IEEE GNSS+R 2019 workshop in Benevento, Italy, in May 2019 covered
reflectometry using satnav and other signals of opportunity (GNSS+R
workshops are held every other year), image credit: ESA

Reflectometry reaches space

“It’s been fantastic to
have experimental evidence, and that’s really been made possible
by the growing availability of smaller satellites,” explains
Manuel. “Because satellite reflectometry is a passive form of
remote sensing, it makes for an attractive potential payload because it
doesn’t need a lot of power to operate. Then one of the results
is meteorology data that private companies intend to make money with by
delivering to public agencies.”

Back in
2003, the UK-DMC microsatellite was the first mission to fly a
reflectometry payload, followed in recent years by, for example, the
UK’s TechDemoSat-1, NASA’s CyGNSS constellation to monitor
hurricanes and the Spire Global constellation of commercial
nanosatellites.

“These satellites have really
given the reflectometry community a wealth of signals, demonstrating
what reflections look like over different surfaces including sea ice,
forests, and even inland water bodies such as the Amazon River and its
tributaries.

“In parts of the ocean near
continental masses and within atolls we are seeing reflected signals
from very calm waters which resembled a mirror, giving us very high
precision down to 1 cm level. Such measurements could potentially
complement current altimetry missions, by for instance measuring sea
level rise.”

ESA activities taking flight

ESA meanwhile is active on reflectometry in various ways, having developed and tested a steerable airborne antenna called the SPIR
(Software PARIS Interferometric Receiver), capable of steering separate
antenna beams to build up a rapid surface picture, and differentiating
between different signal sources, such as GPS from Galileo.

Manuel adds: “ESA’s GNSS
Science Support Center, based at the Agency’s European Space
Astronomy Center near Madrid, has been taking a keen interest in these
activities.”

Missions are also in development, including a dedicated 3U CubeSat with RUAG-Austria and the University of Graz called PRETTY (Passive REflecTomeTry and dosimetry), which would also carry a radiation detector), and a small satellite pair called FSSCat
from Spain’s Universitat Politècnica de Catalunya
(Barcelona), backed through the Copernicus Masters competition, seen as
a prototype for a future reflectometry constellation.

For GNSS reflectometry, the
reflected signal is typically correlated with a clean replica generated
on-board of the spacecraft. The ESA PRETTY CubeSat mission, however,
will correlate the received reflected signal with the received direct
signal. This technique is known as the interferometric approach. The
main advantage for the interferometric approach is, that one is not
bound to use known signals but can exploit signals with unknown data
modulation, opening up the possibility to use more generic signals for
Earth observation. PRETTY will focus on low elevation angles, whereby
the direct and reflected signal will be received via the same antenna. 5)

Figure 7: Preparing a December
2015 flight test of a precursor of a steerable airborne antenna called
the SPIR (Software PARIS Interferometric Receiver), capable of steering
separate antenna beams to build up a rapid surface picture, and
differentiating between different signal sources, such as GPS from
Galileo. Rouhe Erkka (left) pilot from Aalto University (Helsinki,
Finland), Fran Fabra (center) and Serni Ribó (right) from
CSIC-IEEC (Institute of Space Studies of Barcelona, Spain), SPIR
developers (image credit: ESA, M. Martin-Neira)

Figure 8: Upward and downward
antenna radome on the Aalto Skyvan aircraft housing ESA's steerable
airborne antenna called SPIR (Software PARIS Interferometric Receiver),
developed by CSIC-IEEC. The upward antenna detects the original satnav
signal and the downward antenna its surface-reflected equivalent,
comparing the two using interferometry to acquire altimetry and other
data about Earth's land and sea surfaces (image credit: ESA, M.
Martin-Neira)

ESA’s Directorate of Telecommunications and Integrated Applications is also working with the Spire Global company
of San Francisco to fly enhanced reflectometry instruments, starting at
the end of this year. Each Lemur nanosatellite carries an AIS
(Automatic Identification System), GNSS-RO (GNSS-Radio Occultation),
and an ADS-B (Automatic Dependent Surveillance-Broadcast) receiver.

When it comes to the thriving state
of today’s reflectometry community, Manuel recalls the patenting
of his idea as a turning point: ‘Having had this idea, which was
not particularly well received, the proposal by ESA’s Patents
Group to patent it made all the difference. It gave me a feeling of
confidence, that somebody else at least saw the potential of this idea
– and the rest is history.”

Figure 9: Pioneer Spire Global's
nanosatellite in RF test chamber. One of Spire's Satellite
Manufacturing Technicians (Tomasz Chanusiak) tests the Radio Frequency
capabilities of a Lemur-2 nanosatellite in Spire Global's cleanroom in
Glasgow, Scotland. As part of ESA’s ARTES Pioneer program, Spire
Global will aim to prove the value of using nanosatellites for
spaceborne Radio Occultation: the process of using satellites to
measure how GNSS signals are refracted by the Earth’s atmosphere
for weather forecasting and climate change monitoring (image credit:
Spire Global)

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).